1. Field of the Invention
The present invention relates to a ground fault detection system, and more particularly to, a ground fault detection system and method for an inverter which can detect a ground fault generated between an output terminal of the inverter and the earth, and discriminate the phase in which the ground fault has been generated.
2. Description of the Background Art
In general, an inverter is a driving apparatus for a variable speed motor which can efficiently control a speed and torque of the motor, by receiving power from a utility power supply, converting the power into a DC, converting the DC into a variable voltage variable frequency AC, and supplying the AC to the motor. The inverter reduces energy consumption and improves quality by precisely controlling the speed of the motor. Therefore, the inverter has been widely used for automatic facilities such as various air blowers, pumps, machine tools and fiber tools.
On the other hand, distribution line systems using the inverter perform grounding to prevent overcurrent damages from occurring by fault contacts of high and low voltages. For example, in order to prevent overcurrent, a neutral point is grounded in 3-phase Y connection, and one of RST phases is used as a ground in 3-phase Δ connection. However, if an output line of the inverter contacts the earth due to lightening or aging, a large amount of current flows through the earth via a rectifier diode and a switching element of the inverter, which causes secondary accidents such as element damages and fire. Accordingly, a protection system for handling a ground fault generated between the output terminal of the inverter and the earth is required to form the control system using the inverter.
FIG. 1 is an exemplary diagram illustrating an equivalent circuit in a state where a ground fault has been generated in driving an AC motor.
Referring to FIG. 1, in a conventional system for receiving per from an AC power supply 1 and driving an AC motor 5 through an inverter, when R phase of RST phases is used as a ground, if U phase outputted from the inverter contacts the earth to cause a ground fault, a leakage current iG flows through a ground resistor 6. The leakage current iG influences the system through various paths, such as a switching element 4, a rectifier diode 2, a filter condenser 3 for smoothing a DC voltage, and the AC motor 5.
FIG. 2 is a block diagram illustrating a conventional ground fault detection system for detecting and overcoming a ground fault.
As illustrated in FIG. 2, the conventional ground fault detection system includes a (V/f PWM) inverter 9 for converting a DC into an AC and outputting the AC, a current detector 10 for detecting currents flowing through U, V W phases of an output terminal of the inverter 9, a main controller 7 for calculating a ground fault current on the basis of the currents detected by the current detector 10, and determining a failure or a continuous operation, and a variable voltage/variable frequency PWM controller 8 for embodying switching patterns for driving the inverter 9.
A conventional ground fault detection method will now be described.
FIG. 3 is a flowchart showing sequential steps of the conventional ground fault detection method.
As shown in FIG. 3, the main controller 7 sets a ground fault current level as a reference value for estimating a ground fault, and also sets a ground fault counter value as a reference value for deciding the ground fault (ST10).
When the inverter 9 starts outputting, the current detector 10 detects the currents of each phase flowing through the output terminal of the inverter 9 (ST11). The main controller 7 compares a current value obtained by adding up the currents of each phase flowing through the output terminal of the inverter 9 with the set ground fault current level (ST13). If the calculated ground fault current is greater than the set ground fault current level, the main controller 7 increases an inside counter (ST14).
Finally, if the added counter value is larger than a preset ground fault counter value, the main controller 7 processes the ground fault (ST15 and ST16).
FIG. 4 is a waveform diagram illustrating a general ground fault detection result.
As depicted in FIG. 4, channel 1 indicates a U phase current, and channel 2 indicates a ground fault current. Here, the experiment has been made by equalizing a frequency of an AC power supply to an output frequency of an inverter, and operating the inverter. That is, although the phase current is seriously distorted due to an external ground fault and a large amount of ground fault current flows, the conventional ground fault detection method cannot always handle the ground fault.
As a result, in the conventional ground fault detection system, in case the ground fault is generated after power supply and the ground fault current flows over an overcurrent trip, the system can be protected by an overcurrent protection circuit. However, if the ground fault current does not reach the overcurrent level, the motor is damaged. In addition, when the power frequency of the inverter is similar to the output frequency, the ground fault is not detected. Accordingly, in the conventional ground fault detection system, the ground fault current flows through the earth via the rectifier diode and the switching element of the inverter, which causes secondary accidents such as element damages and fire and reduces reliability of the product. As input voltages of an induction motor overlap with each other, torque unbalance is generated and properties of a predetermined speed of the AC motor are deteriorated.